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Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 3149
Maxim > Design Support > Technical Documents > Application Notes > Temperature Sensors and Thermal Management > APP 3149
Keywords: PWM, buffer, convert, amplify, amplifier, fan, speed, controlller
APPLICATION NOTE 3149
Circuit Converts PWM to Amplified and Buffered
Linear Signal
Jun 16, 2004
Abstract: A simple circuit converts a low-voltage PWM signal to an amplified and buffered linear output.
Intended for fan speed control, it allows a 3.3V input to provide linear control of a 12V fan.
Maxim makes a variety of fan speed controllers with PWM outputs to control fan speed as a function of
temperature. These work by cycling the power to the fan off and on. The fan's speed with the duty cycle
setting the fan's speed. In many situations the typical application circuit for these is adequate. However, some
situation require a constant power supply to the fan due to audible noise from the fan modulation.
If the noise from cycling the fan's power is too much, consider the circuit of Figure 1. In this case,
complementary pair of BJTs (Q1) and a PMOS FET (Q2) create a linear amplifier.
Figure 1. A simple circuit converts a low-voltage PWM signal to an amplified and buffered linear output.
The circuit works in the following manner. The base of the PNP in Q1 is the non-inverting input to the
amplifier, and the emitter of the NPN is the inverting input. The PNP is biased as an emitter follower, and the
NPN is used both as an emitter follower and as the initial gain element. As the PNP and the NPN operate at
roughly the same current density and temperature, the two input voltages approximately track each other. Any
current which flows out of the inverting input is mirrored to the NPN collector, causing a drop across resistor
R2. The drop across R2 drives the V
GS
of Q2, which is amplified at Q2's drain, the amplifier output. The
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